Hip replacement surgery typically is performed to compensate for severe damage of the acetabulum due to disease, trauma or other factors, and includes the steps of removing all or part of the existing joint and substituting for the removed bone a femoral component attached to the patient's femur and an acetabular cup attached to the patient's acetabulum. The prosthetic acetabular cup is implanted to substitute for the socket of the hip joint, and is mated with a prosthetic femoral component to complete the hip replacement surgery. In order to achieve optimal performance of the combined acetabular and femoral prostheses, the acetabular cup must be properly positioned in the acetabulum. An improperly positioned acetabular component can lead to dislocations of the hip joint, decreased range of motion, and eventual loosening or failure of one or both of the acetabular and femoral components.
Acetabular cups may be formed of a metal, ceramic or plastic. Incorrect acetabular component positioning can lead to edge loading and undesirable effects across bearings composed of any material, such as dislocation, increased wear, ceramic squeaking, elevated metal ion release and fractures. Studies of post-operative cup placement demonstrate that seating the cup in particular orientations provides for improved wear patterns compared with seating the cup in other orientations. For example, a study by Langton et al., entitled “The Effect of Component Size and Orientation on the Concentrations of Metal Ions after Resurfacing Arthroplasty of the Hip”, and published in the Journal of Bone Joint Surgery (2008; 90-B:1143-51) demonstrates that the version angle (as measured on EBRA software) may influence wear. To improve the wear of the cup and the product performance of the hip system, a surgeon endeavours to seat the cup in an orientation that is predicted to provide good wear patterns in accordance with the post-operative studies.
In preparation for surgery, the patient is x-rayed in the same two planes as those provided for in the post-operative studies. The surgeon then uses the x-rays as a means of preliminarily planning the size of the acetabular and femoral prostheses and the position of each when surgery is complete. During surgery, the surgeon uses a reamer to remove bone to form a hemispherical shape in the acetabulum. The reamed acetabulum enables a nearly unlimited number of angular cup positions as the cup may be seated at any angle within the hemisphere.
Next, the surgeon attaches the cup to an inserter/impactor and attempts to position the cup in a way that approximates the planned-for angles in the pre-operative plan. Surgeons often have difficulty in performing this step as he or she must manipulate the cup in three dimensions while attempting to correct for angular changes that occur when translating the pre-operative radiographic angle into operative angles. Thus, a common cause of malpositioning is the difference between the radiographic angles provided in the pre-operative plan and the operative angles observed by the surgeon.
FIG. 1 depicts the difference in the projection of angles seen from different views. For example, 45° of inclination and 30° of anteversion achieved in the operative environment will provide a steeper inclination angle of 50° when displayed on an A/P radiograph. To achieve a position that will provide a 45° inclination angle on the A/P radiograph, the surgeon must adjust the operative inclination angle. Thus, to succeed in positioning the cup at a desired angle, the surgeon needs to translate the information from the post-operative studies generated in two dimensions at a first viewing angle into data that is useful when in the operating theatre, where the surgeon operates in three dimensions and observes the patient at a viewing angle that is not the viewing angle provided in an A/P radiograph.
To assist surgeons in accurately locating and aligning prostheses and instruments, surgeons commonly rely on instruments. Inclination and version guides are widely used during total hip arthroplasty to assist in aligning the acetabular cup. Separate guides may be provided for inclination and version angle. Alternatively, a combined guide may be provided. One known form of combined alignment guide enables the position of the acetabular cup to be set at an angle relative to the floor of the operating room and the long axis of the patient. Such an alignment guide (that provides a measure of operative angles to position the cup inserter in surgery) does not directly represent what the surgeon will see post-operatively on an A/P radiograph. As discussed earlier, all of the clinical analysis on cup wear has been based on post-operative measurements. As a result, the surgeon aims to position the cup to match as closely as possible the desired position as shown in a post-operative radiograph of the cup. The disadvantage of using operative angles in surgery is that, as you vary operative anteversion, the radiographic inclination angle of the cup changes.
It is an object of embodiments of the present invention to obviate or mitigate one or more of the problems associated with the prior art, whether identified herein or elsewhere.